Add reference to section about adding a hints file to all exercises
with difficulty 5.

* Add hints to exercises without starter implementation

As exercises with difficulty 5 are the first exercises without starter
implementation, they should have a hints file explaining how to fix the
compiler errors that occur when an exercise doesn't have any starter
implementation.

This should be detailed in the policies doc.

* flatten-array: regenerate README

Also update description for how to generate readme as it was lacking
detail.